This invention relates to an exhaust gas purifier for automotive engines and, more particularly, to an exhaust gas purifier for a diesel engine of the class wherein a throttle valve is provided in an intake passage for reducing the flow rate of intake air during engine idling.
The vibration and noise produced by a diesel engine are generally of a higher level than those of gasoline engines due to the diesel engine's high compression ratio. Such vibration and noise are disadvantageous in terms of driver and passenger comfort, particularly during engine idling.
In order to reduce engine vibration and noise during idling, there is known in the prior art a diesel engine in which a throttle valve is provided in an intake passage. During engine idling, the throttle valve is closed in response to the release of the accelerator pedal of the vehicle, thereby reducing the amount of the air drawn into the cylinders. With a reduced amount of intake air being compressed at the compression stroke, engine vibration and noise are suppressed to a considerable extent. However, such a diesel engine suffers from the shortcoming that the total amount of unburned hydrocarbon emission is increased due to air throttling.